1. Field of the Invention
The invention relates to an overvoltage protection element with a housing, with at least one overvoltage limiting component which is located in the housing, especially a varistor, with two terminal contacts for electrical connection of the overvoltage protection element to the current path or signal path to be protected, with an electrically conductive connecting element and with a spring system which acts on the connecting element, the first terminal contact being in electrically conductive contact directly with the first pole of the overvoltage limiting component, in the normal state of the overvoltage protection element the connecting element being in electrically conductive contact both with the second terminal contact and also with the second pole of the overvoltage limiting component via a thermally separating connection which separates when the temperature of the overvoltage limiting component exceeds a given boundary temperature, and with the thermal connection separated the connecting element being moved out of the contact position by the force of the spring system such that the connecting element no longer has electrically conductive contact with the second terminal contact and the second pole of the overvoltage limiting component.
2. Description of Related Art
German Patent DE 42 41 311 C2 discloses an overvoltage protection element which has a thermal disconnector for monitoring the state of the varistor. In this overvoltage protection element, the first terminal element is directly connected via a flexible conductor to a rigid disconnecting element whose end facing away from the flexible conductor is connected via a solder site to the terminal lug provided on the varistor. The other terminal element is permanently connected via a flexible conductor to the varistor or a terminal lug on the varistor. The disconnecting element is exposed to a force from a spring system which leads to the disconnecting element being moved linearly away from the terminal lug when the solder connection is broken so that the varistor is electrically isolated when thermal overloaded. By way of the spring system, when the solder connection is broken, a telecommunications contact is activated at the same time so that remote monitoring of the state of the overvoltage protection element is possible. The level of the maximum allowable impulse current which can be discharged by the overvoltage protection element is limited by the use of the flexible conductors to connect the varistor.
German Utility Model DE 20 2004 006 227 U1 and corresponds to U.S. Pat. No. 7,411,769 B2 discloses an overvoltage protection element in which the monitoring of the state of a varistor takes place according to the principle of a temperature switch so that when the varistor is overheated, a solder connection between the varistor and the disconnecting element is broken; this leads to electrical isolation of the varistor. Moreover, when the solder connection is broken, a plastic element is pushed by the reset force of a spring out of a first position into a second position in which the disconnecting element, which is made as an elastic metal tongue, is thermally and electrically separated from the varistor by the plastic element so that an arc which may be present between the metal tongue and the contact site of the varistor is extinguished. Since the plastic element has two colored markings located next to one another, at the same time, it acts as an optical state display so that the state of the overvoltage protection element can be read directly on site.
European Patent Application Publication EP 0 716 493 A1 discloses an overvoltage protection element with two varistors, which has two disconnecting means which can individually isolate the varistors on their live end. The disconnecting means each have an elastic disconnecting tongue, the first end of the disconnecting tongue being permanently connected to the first terminal and the second end of the disconnecting tongue, in the normal state of the overvoltage protection element, being attached to the connecting tongue on the varistor via a solder site. If undue heating of the varistor occurs, this leads to melting of the solder connection. Since the disconnecting tongue in the soldered-on state (normal state of the overvoltage protection element) is deflected out of its rest position, and is thus pretensioned, the free end of the disconnecting tongue springs away from the connecting tongue of the varistor when the solder connection softens, by which the varistor is electrically isolated. To ensure the required insulation and tracking resistance and to extinguish an arc which forms when the gap opens it is necessary that, when the disconnecting tongue is pivoted, a distance between the second end of the disconnecting tongue and the connecting tongue of the overvoltage limiting component is achieved that is as large as possible. Moreover, the cross section of the disconnecting tongue should not be too large so that it has a sufficient spring property. However, this likewise leads to limitation of the maximum allowable impulse current.
European Patent Application Publication EP 09 87 803 A1 discloses an overvoltage protection element of the initially described type. In this overvoltage protection element, one end of a rigid, spring-loaded slide, in the normal state of the overvoltage protection element, is soldered both to the first terminal element and also to a terminal lug which is connected to the varistor. Here, undue heating of the varistor also leads to heating of the solder site so that the slide is withdrawn from the connecting site between the first terminal element and the terminal lug as a result of the force of a spring acting on it; this leads to electrical isolation of the varistor. To extinguish an arc which forms when the gap opens, only a air clearance between the first terminal element and the terminal lug connected to the varistor is available so that the overvoltage protection element must have relatively large dimensions in order to be able to reliably and promptly extinguish the arc.
The known overvoltage protection elements are generally made as “protective plugs” which form an overvoltage protection device together with the bottom part of the device. For installation of such an overvoltage protection device which, for example, is designed to protect the phase-routing conductors L1, L2, L3 and the neutral conductor N, and optionally, also the ground conductor PE, there are corresponding terminals for the individual conductors on the bottom part of the known overvoltage protection devices. For simple mechanical and electrical contact-making of the bottom part of the device with the respective overvoltage protection element, in the overvoltage protection element, the terminal contacts are made as plug pins for which there are corresponding sockets which are connected to the terminals in the bottom part of the device, so that the overvoltage protection element can be easily plugged onto the bottom part of the device.
In these overvoltage protection devices, installation and mounting can be carried out very easily and in a time-saving manner due to the capacity of the overvoltage protection elements to be plugged in. In addition, these overvoltage protection devices, in part, still have a changeover contact as the signaler for remote indication of the state of at least one overvoltage protection element and an optical state display in the individual overvoltage protection elements. The state display indicates whether the overvoltage limiting component which is located in the overvoltage protection element is still serviceable or not. Here, the overvoltage limiting component is especially varistors, but depending on the application of the overvoltage protection element, gas-filled surge arresters, spark gaps or diodes can also be used.
The above described thermal disconnectors which are used in the known overvoltage protection elements and which are based on melting of a solder connection must perform several functions. In the normal state of the overvoltage protection element, i.e., in the unbroken state, a reliable and good electrical connection between the first terminal element and the overvoltage limiting component must be ensured. When a certain boundary temperature is exceeded, the gap must ensure reliable isolation of the overvoltage limiting component and continuous insulation resistance and tracking resistance. Moreover, if the overvoltage protection elements are to have dimensions that are as small as possible, so that the overvoltage protection devices do not exceed the dimensions given for the mounting rail devices, this leads to the known overvoltage protection devices being able to be used only in the lower and the middle power classes, i.e., for pulse currents≦65 kA.